1. Field of the Invention
The present invention relates to a pinion of a starter motor employed in an internal combustion engine for a motor vehicle or the like. More particularly, the invention is concerned with a pinion of a starter motor for an internal combustion engine which pinion can be implemented in a small size and light weight.
2. Description of Related Art
For having better understanding of the invention, description will first be made in some detail of the technical background by reference to FIGS. 4, 5 and 6, in which FIG. 4 is side elevational view showing partially in section a hitherto known or conventional starter motor for an internal combustion engine disclosed, for example, in Japanese Unexamined Patent Application Publication No. 37785/1995 (JP-A-7-37785), FIG. 5 is an enlarged side view showing in section a pinion incorporated in the starter motor shown in FIG. 4, and FIG. 6 is an end view of the same as viewed in the axial direction in FIG. 4.
Referring to the figures, a starter motor (also known as the cranking motor) employed for starting an internal combustion engine (not shown) is provided with an electromagnetic switch unit 1 mounted on the starter motor for driving a plunger 2 in the axial direction.
A lever 3 interlocked with the plunger 2 has a tip end portion adapted to engage with the plunger 2 and a bottom end portion adapted to engage with a rear end face (end face opposite to an operative end face) of an overrunning clutch unit 4, as will be described later on. The lever 3 is mounted rotatably about a supporting member 3a functioning as a fulcrum.
The starter motor includes an armature 5 driven rotationally and an output shaft 6 formed as an integral part extending from a rotatable shaft of the armature 5. Formed in an outer periphery of the output shaft 6 is a helical spline (not shown) which meshes with the overrunning clutch unit 4 so that the latter can slideably move along the output shaft 6, being guided by the helical spline.
A pinion 7 adapted to mesh with a ring gear (not shown) of the internal combustion engine (not shown either) is operatively coupled with the overrunning clutch unit 4 such that the pinion 7 can rotate only in one direction (i.e., the direction in which the internal combustion engine is started).
A plurality of gear teeth 7G are formed in an outer peripheral surface of the pinion 7 so as to mesh with the ring gear of the engine while a bearing 8 is snugly mounted on an inner periphery of the pinion 7. In this conjunction, it should be added that the pinion 7 is adapted to move slideably in the axial direction along the output shaft 6.
Next, description will be directed to the operation of the pinion of the conventional starter motor for the internal combustion engine described above.
When a key switch (not shown) of the internal combustion engine is manually closed by a driver, an exciting coil (not shown) of the electromagnetic switch unit 1 is electrically energized, whereby the plunger 2 is attracted under a magnetic force to move rearwardly (i.e., toward the left-hand side as viewed in FIG. 4).
As the plunger 2 moves, as mentioned above, the lever 3 rotates around the supporting member 3a serving as the fulcrum, as a result of which the overrunning clutch unit 4 is forced to move forwardly (to the right-hand side as viewed in FIG. 1).
Consequently, the pinion 7 coupled with the overrunning clutch unit 4 moves forwardly together with the overrunning clutch unit 4 along the output shaft 6 with the gear teeth 7G being brought to the state in which it meshes with the ring gear of the internal combustion engine.
Furthermore, as the plunger 2 moves under the attraction of the electromagnetic switch unit 1, as mentioned above, a movable contact (not shown) of the electromagnetic switch unit 1 moves to bear against a stationary contact (not shown) disposed at a rear position.
In this way, the armature 5 is electrically energized to be thereby rotated to generate a torque which is transmitted to the pinion 7 by way of the helical spline and the overrunning clutch unit 4 and finally to the internal combustion engine by way of the ring gear which meshes with the gear teeth 7G of the pinion 7.
As is apparent from the above, the internal combustion engine is started through the medium of the pinion 7 designed to mesh with the ring gear. In this conjunction, there has arisen a demand for small-size implementation or miniaturization of the starter motor in recent years in view of installation of an increased number of peripheral auxiliary machines for the internal combustion engine of the motor vehicle and reduction in the fuel cost.
As the method for the realization of miniaturization of the starter motor, it is conceivable to increase the gear ratio between the pinion 7 and the ring gear to thereby reduce the weight of the starter motor by a proportion corresponding to the increment of the gear ratio.
As the practical means for increasing the gear ratio, there may be mentioned two methods, i.e., first method of increasing the number of teeth of the ring gear and second method of decreasing the number of the gear teeth 7G of the pinion 7.
However, the first method is disadvantageous in that the diameter of the internal combustion engine as well as that of the transmission increases as the number of the teeth of the ring gear increases, involving the necessity for increasing the available space of the engine room of the motor vehicle while taking into account the layout of the components therein, which in turn means that the weight of the motor vehicle will ultimately be increased.
On the other hand, the second method can not realize any sufficient reduction of the weight because limitation is inevitably imposed on the decrease of the gear teeth 7G of the pinion 7 in view of the fact that the strength of the output shaft 6 as well as that of the tooth bottom portion of the pinion 7 is thereby lowered.
In general, for the pinion 7 employed in the starter motor, at least eight teeth are required in case the module is 2.54 (corresponding to the diametrical pitch of "10") in order to ensure an adequate strength while at least 9 teeth are required in case the module is 2.117 (corresponding to the diametrical pitch of "12"). In this conjunction, the module is defined as a quotient derived by dividing the diameter of the pitch circle of the pinion by the number of teeth.
Parenthetically, the applicable value of module is determined in dependence on the specifications of the ring gear. In the case of the pinion 7 shown in FIG. 6, it is assumed that the pinion 7 is provided with nine teeth with the module being 2.117 (corresponding to the diametrical pitch of "12").
For the reasons mentioned above, when the pinion 7 is to be implemented with a lesser number of teeth than the lower limit mentioned above, it is then required to realize at least the output shaft 6 in a smaller diameter or decrease the thickness of the bottom portions of the teeth of the pinion 7, which however means that the pinion 7 or the output shaft 6 may be injured under excessively large load or shock applied to the pinion 7.
As will now be appreciated from the foregoing, the pinion of the starter motor for the internal combustion engine known heretofore suffers a problem that the weight of the pinion 7 can not be decreased without impairing the strength thereof because the pinion 7 is so formed that the inner diameter thereof is to be constant in the axial direction.